1. Field of the Invention
The present invention relates to a wheeled working machine such as a wheeled crane having a steering apparatus.
2. Description of the Related Art
A steering apparatus for a wheeled crane, having an upper rotating body rotatably mounted on a lower traveling body including front wheels and rear wheels, includes several full-hydraulic systems as disclosed in Japanese Unexamined Patent Application Publication No. H5-39057, which includes four steering cylinders for steering the front and rear wheels, respectively, whereby an operator in a cab steers at least one of the front wheels and the rear wheels by introducing hydraulic oil to predetermined steering cylinders in accordance with a selection mode, such as a rotational amount of a steering handle and a switch operation, which are operated by the operator. In this case, the selection mode includes a normal mode for steering only the front wheels, a crab mode for steering the front and rear wheels in the same direction, a clamp mode for steering the front and rear wheels in opposite directions to each other for reducing a turning radius, and a reverse steering mode for driving in the same feeling as that in normal traveling even when traveling in a state that the upper rotating body is rotated at 180° to the lower traveling body.
However, since in this apparatus, the steering handle and the wheels are not mechanically connected together for transmitting a steering force, the traveling speed during traveling on a general public road is limited worldwide; it is limited to 25 km/h or less especially in Europe.
On the other hand, a steering apparatus is known in Europe in that the steering handle and a tie rod for changing the direction of the front wheels are mechanically connected together as a wheeled crane capable of traveling at a high speed of about 80 km/h. This apparatus includes steering cylinders for changing the direction of the rear wheels so that the rear wheels can be steered by the switch operation independently from the rear wheel steering by the handle operation, and the clamp mode and the crab mode can be selected in addition to the normal mode.
However, when traveling in the cramp or crab mode, this apparatus has a disadvantage of the poor operationality because the rear wheel steering must be simultaneously executed by the switching operation independently from the front wheel steering by the handle operation. The apparatus also has a disadvantage that since the steering handle on the upper rotating body and the tie rod of the lower traveling body are mechanically connected together, when the upper rotating body is rotated during crane operation, the steering handle rotates on its own accord, giving the operator unpleasant feeling and a disadvantage of impossible to travel in the reverse steering mode.